The present invention relates generally to battery charging systems, and, more particularly, to a battery charging system for accurately determining battery charge states of a plurality of batteries.
One or more batteries are frequently utilized as a power source to power an electrical device. In some instances, the use of a battery to power an electrical device is necessary to permit operation of the electrical device when the electrical device is not, or cannot be, positioned proximate to a permanent power supply.
A battery, however, can store only a finite amount of energy, and, after a period of use, the battery becomes discharged, necessitating replacement thereof to permit continued operation of the electrical device.
A battery is, for instance, essential to operate a portable or transportable radiotelephone. When operative to transmit or to receive an information signal, the radiotelephone dissipates power at a rate of approximately three watts. A battery coupled to power the radiotelephone when operative to dissipate energy at a rate of three watts, may become discharged in one hour, or even sooner if the battery is initially not fully charged. Once discharged beneath a certain level, the battery coupled to the radiotelephone must be removed and replaced with a new battery.
A rechargeable battery is advantageous for the reason that, after the battery has been discharged, the rechargeable battery may be recharged and resued. The same rechargeable battery may be recharged and reused up to, or even in excess of, five hundred times.
Battery charging apparatus exists and is readily available for recharging one or more rechargeable batteries. Typically, the battery charger is comprised of a support structure for supporting a plurality of batteries, and a current source for charging the batteries when suitably positioned upon the support structure and connected to the current source to permit charging thereof. Application of a charging current generated by the current source to the batteries for a period of time recharges the batteries. The amount of time during which the charging current must be applied to the batteries is dependent upon the battery type, the extent to which the individual ones of the batteries have been discharged, and the magnitude of the charging current applied to the batteries.
Some types of battery charging apparatus supply a relatively small current (e.g., a C/10 value wherein c is defined to be the one-hour capacity of a battery) to the batteries, and other types of battery charging apparatus supply a significantly larger current (e.g., a C value) to the batteries. The process of applying a large current to the batteries is referred to as fast charging of the batteries. Still other types of battery charging apparatus permit either the relatively small current or the fast charging current to be supplied to the battery.
For reasons of convenience, applying the fast charging current to the batteries is advantageous. However, only certain types of batteries may be charged with a fast charging current. Nickel-cadmium type (Ni-Cd) batteries are one type of rechargeable battery which may be charged with the fast charging current. Additionally, even when the battery construction is of a type permitting application of a fast charging current thereto, care must be exercised to ensure that application of the fast charging current to the battery is terminated once the battery has been fully charged. Overcharging of a battery can cause gassing, electrolytic venting of the battery, permanent loss of the battery capacity and actual physical damage to the battery.
Therefore, battery charging apparatus of constructions which permit fast charging of one or more rechargeable batteries regularly include means for automatically terminating application of the fast charging current to the battery (or batteries) once the battery has been fully charged. For instance, all of the following methods are known for determining when a battery has been fully charged: coulometric control, time control, pressure sensing, temperature sensing, incremental temperature cut-off, differential temperature cut-off, rate of temperature change, voltage magnitude sense, positive rate of voltage change, inflection point cut-off, voltage decrement cut-off, and negative voltage change. Additionally, U.S. patent application Ser. No. 519,532 filed on May 4, 1990, and entitled "System For Fast Charging for a Battery" to Goedken, et al. discloses a fast charge termination technique wherein both the first and the second derivatives of voltages measured across output terminals of a battery are calculated. Only when both the first and the second derivatives of the voltages measured across the output terminals of the battery are negative values is the fast charging of the battery terminated.
Several battery charger constructions permit recharging of a plurality of batteries when each of the batteries are positioned in a parallel electrical connection therebetween. Application of the fast charging current to the batteries positioned in the parallel connection therebetween causes current to be applied to each of the batteries. However, individual ones of the batteries may become fully charged at different times due, for example, to variances of battery characteristics of particular ones of the batteries and the extent to which particular ones of the batteries have been discharged prior to charging thereof. Such battery charger constructions are therefore of limited usefulness for fast charging batteries. When a first of the batteries positioned in parallel becomes fully charged, fast charging thereof should be terminated to prevent damage to that battery. Termination of fast charging, however, results in incomplete charging of the other batteries positioned in parallel with the fully charged battery. Battery charger constructions in which the batteries are positioned in parallel are most frequently utilized when only a slow charging current is applied to the batteries. Because battery overcharging is not a significant problem when a slow charging current is applied to a battery, termination of charging of all of the batteries is not required once one of the batteries becomes fully charged.
More frequently, battery charger constructions which permit fast charging of a plurality of batteries sequentially charge each of the batteries. That is, the fast charging current is applied to only one battery at a time. Once a first battery has been fully charged, fast charging of that battery is terminated, and the fast charging current source is connected to a second of the batteries. In order to fully charge each of the plurality of batteries positioned to permit recharging thereof, the fast charging current is applied to each of the batteries in sequence. The amount of time required to fully charge the plurality of batteries is dependent upon the number of batteries positioned for recharging thereof and the extent to which the batteries have been discharged.
Some battery chargers provide some type of indication of the charge states of the batteries positioned thereat for recharging. In order to indicate the charge states of the batteries, the charge states of the batteries must first be determined. An accurate determination of the charge state of a battery requires application of a current to a battery for a period of time. Because sequential battery chargers sequentially apply a charging current to the batteries, most existing sequential battery chargers provide an indication of the charge state of only the battery undergoing charging thereof.
What is needed, therefore, is a means for accurately determining the charge states of every battery positioned at a sequential battery charger.
In many instances, electrical devices are designed such that only one battery is required to operate the electrical device. For instance, a conventional radiotelephone is typically operated by a single battery. After operation of the radiotelephone, the battery becomes discharged, necessitating replacement thereof. Once removed from the radiotelephone, and positioned to be charged by battery charger apparatus, the radiotelephone user is usually more interested in quickly recharging one battery, as only one battery is required for operation of the radiotelephone, rather than waiting a greater period of time while a plurality of batteries are recharged.
Therefore, attempts have been made to provide battery charging apparatus to charge first a particular one of a plurality of batteries. Most simply, a certain position, such as a first charging pocket formed of a portion of a battery charger, may be defined as a "priority charging location". A battery positioned at the priority charging location is charged before any other battery. Only when the battery positioned at the priority charging location is fully charged, are any of the other batteries connected to the fast charging current source.
U.S. Pat. No. 4,849,682 discloses a battery charging system for sequentially charging a plurality of batteries. According to the disclosed system, once the batteries are positioned at a battery charger to permit recharging thereof, each battery is checked, or "interrogated", to determine the level of charge (i.e., the charge state) of the battery. The battery having the highest level of charge (i.e., charge state) is assigned a highest priority, and is the battery which is first connected to the supply of fast charging current to charge the battery thereby. The temperature of the battery may additionally be measured, and only the battery which is of the highest charge state, and is of a temperature within a range suitable for charging thereof, is assigned highest priority and connected to the fast charging current source. The battery charging system further discloses a means for delivering a higher than rated, fast charging current source to the battery assigned highest priority for a subinterval of a time interval. The current average of the fast charging current applied to the battery assigned the highest priority, however, corresponds to the rated fast charging current to be applied to the battery. An equalizer current is supplied to a second of the batteries positioned to permit charging thereof during other subintervals of the time interval.
The system, however, is based upon the assumption that, upon positioning of the batteries at the battery charger to permit charging thereof, the actual charge states of the battery are immediately determinable. In fact, upon such initial checking or "interrogation" of the batteries, the batteries oftentimes exhibit charge levels which are only transitory or which may be indicative of more than one charge state. Hence, the determined charge states oftentimes do not accurately represent the actual charge state of the battery. The disclosed system may, therefore, assign highest priority to fast charge a battery which is actually of a lower charge state than another one of the batteries. Such a determination prevents a fastest possible charging sequence for recharging a single battery of a plurality of batteries.
What is needed, therefore, is a battery charging system which accurately determines battery charge states of a plurality of batteries to determine thereby an optimal battery charging sequence for sequentially charging a plurality of batteries.